1. Field of the Invention
The present invention relates to an output circuit for outputting a voltage corresponding to a charging voltage of a capacitor to a load.
2. Related Art
For instance, a communication signal output circuit used on board of a vehicle is designed to output a trapezoidal wave signal with low slew rate in leading and trailing edges for the purpose of reducing radio noise by suppressing induction radiation from communication lines installed in the vehicle. This type of trapezoidal wave generating circuit has been proposed, for instance, in JP-A-6-214665 or JP-A-9-261016. In a circuit of this type, a trapezoidal wave signal is generated by performing charging and discharging at constant current to a capacitor.
FIG. 11 is an electrical block diagram of a trapezoidal wave output circuit commonly in use. In FIG. 11, a trapezoidal wave output circuit 1 using IC is operated at relatively high power voltage (e.g. a voltage of 12 V-16 V outputted from a battery) applied between power terminals 2 and 3. Thus, it is designed as bipolar IC, which can be easily turned to highly pressure-proof compared with MOSIC.
The trapezoidal wave output circuit 1 comprises a charge-and-discharge circuit 4, a drive circuit 5 and an output transistor 6, and a load 8 is connected between an output terminal 7 and a power terminal 3. Among these components, the charge-and-discharge circuit 4 comprises a capacitor 9, constant current circuits 10 and 11 for charging or discharging this capacitor 9, and a switching circuit 13 for switching charge-and-discharge operation according to a switching signal Sa given via a control terminal 12. The drive circuit 5 comprises a 3-stage emitter follower circuit, which contains transistors 14, 15 and 16 and resistances 17, 18 and 19. A base current is supplied to an output transistor 6 via the resistance 19. Diodes 20 and 21 are provided to protect the transistors 15 and 6 in case power source is connected reversely.
In the arrangement as described above, a trapezoidal wave voltage is generated according to the switching signal Sa between two terminals of the capacitor 9. This voltage undergoes sequential level-shift by the transistors 14, 15, and 16 and is turned to a base voltage of the output transistor 6. The output transistor 6 also makes up an emitter follower circuit together with the load 8. Eventually, the trapezoidal wave output circuit 1 outputs the terminal voltage of the capacitor 9 to the load without changing the voltage level.
In this case, electric current is amplified by the drive circuit 5 and the output transistor 6. In order that the output transistor 6 can sufficiently drive the load 8 regardless of the value of the output voltage Vo, a sufficiently high base current must be supplied to the output transistor 6 even in case the highest output current lo flows, i.e. in case the output voltage Vo is at the maximum level. Specifically, if it is supposed that maximum voltage value of the output voltage Vo is Vo(max), resistance value of the load 8 is RL, and DC current amplification rate of the output transistor 6 is hFE, then the current value IB1 to be supplied to the output transistor 6 is given by the equation given below. Because this base current totally flows via the resistance 19, resistance value of the resistance 19 is determined by the current value IB1 and the maximum voltage value Vo(max).
IB1xe2x89xa7(Vo(max)/RL)/hFExe2x80x83xe2x80x83(1)
However, when the output voltage Vo is decreased in association with the discharge of the capacitor 9, end-to-end voltage of the resistance 19 is increased in the same extent as the voltage decrease of the output voltage Vo. The electric current flowing to the resistance 19 is increased to a level higher than the electric current value IB1 as given by the above equation (1). Also, the base current required for driving the output transistor 6 is decreased by the same extent as the decrease of the output current Io. The increment of the electric current flowing to the resistance 19 and the decrement of the base current are not turned to the base current of the output transistor 6, and these are sent to the power terminal 3 via the transistor 16 as useless current. When it is designed in a circuit configuration provided with a constant current circuit with current value of IB1 instead of the resistance 19, the decrement of the base current flows uselessly. As a result, consumption current (i.e. power consumption) of IC is increased, and this leads to the problems such as the increase of chip temperature or deterioration of the battery capacity.
In contrast, in a trapezoidal wave output circuit 22 as shown in FIG. 12, the output transistor 6 and the transistor 23 are connected by Darlington connection, and the base current to be outputted from the drive circuit 24 is decreased. In this case, the drive circuit 24 comprises a 2-stage emitter follower circuit, which contains transistors 14 and 16 and resistances 17 and 19.
According to this trapezoidal wave output circuit 22, the electric current value IB2 to be supplied to the transistor 23 is a value, which is obtained when the current value IB1 is divided by DC current amplification rate of the transistor 23. Even when the output voltage Vo is decreased, the uselessly flowing electric current is decreased. However, the output voltage Vo cannot be higher than a value, which is calculated by: (power voltage VBxe2x80x94forward voltage VF of diode 25xe2x80x94collector-emitter voltage of the transistor 23xe2x80x94base-emitter voltage of the transistor 6). Thus, compared with the trapezoidal wave output circuit 1 as described above, maximum voltage value Vo(max) is decreased.
As a result, current output capacity of the trapezoidal wave output circuit 22 may be decreased, or peak value of the trapezoidal wave communication signal may be in shortage and communication error may occur frequently when the power voltage VB is decreased. Also, the collector-emitter voltage of the output transistor 6 is increased by an amount equal to the voltage VF compared with the case of the trapezoidal wave output circuit 1, and this leads to the increase of collector loss. As described above, in the trapezoidal wave output circuits 1 and 22 of the conventional arrangement, it has not been possible to decrease power consumption and to maintain high output voltage Vo at the same time.
To solve the above problems, it is an object of the present invention to provide an output circuit, by which it is possible to output a voltage corresponding to a charging voltage of a capacitor, and to reduce power consumption while maintaining maximum voltage value to be outputted at high level.
According to the present invention, an output transistor outputs a voltage corresponding to a charging voltage of a capacitor to a load, and electric current corresponding to the output voltage is supplied. The charging voltage of this capacitor is generated by a charge-and-discharge circuit. In general, it is turned to a voltage, which is not a constant voltage but is increased or decreased in similar manner to a trapezoidal wave voltage. According to a drive circuit of this invention, a voltage detection circuit detects the charging voltage of the capacitor, and a variable current circuit supplies a base current corresponding to the detected charging voltage to the output transistor. Specifically, the drive circuit supplies a base current of relatively high value when the output voltage is high and output current of the output transistor is high. When the output voltage is low and the output current of the output transistor is low, a base current of relatively low value is supplied. In this respect, this is different from the drive circuit of the conventional type, in which the base current of the output transistor is determined according to the maximum output voltage only.
As a result, the drive circuit can supply a base current, which is required and sufficient for driving the load to the output transistor, and electric current is not uselessly supplied in the circuit. This makes it possible to reduce consumption current (power consumption) of the drive circuit, and hence, the output circuit. Also, it is possible to prevent deterioration of the capacity of the battery used as power source and to avoid the increase of chip temperature in the IC-using circuit.
Also, the base current is optimized by the use of the voltage detection circuit and the variable current circuit as described above. For instance, there is no need to design the output transistor by Darlington connection to decrease the base current. (Naturally, Darlington connection may be used when it is necessary to supply higher current.) Further, it is possible to maintain the maximum voltage to be outputted.
According to a preferred aspect of the present invention, the voltage detection circuit directly detects the terminal voltage of the capacitor as the charging voltage of the capacitor. As a result, it is possible to accurately detect end-to-end voltage of the capacitor without being influenced from various types of nonlinear circuits in the drive circuit or from temperature change.
Further, according to another preferred aspect of the present invention, the drive circuit comprises an emitter follower circuit. The emitter terminal of each transistor is turned to have a voltage level-shifted by approximately a fixed voltage from the terminal voltage of the capacitor. Thus, the voltage detection circuit can detect the voltage corresponding to the terminal voltage of the capacitor, i.e. the charging voltage of the capacitor.
The emitter follower circuit has high input impedance, and the influence of the drive circuit on the charging of the capacitor is low. Also, the output impedance is low, and there is less influence from the connection of the voltage detection circuit. As a result, the output voltage is turned to a voltage on the same level as the terminal voltage of the capacitor generated by the charge-and-discharge circuit, and voltage distortion is reduced.
According to still another preferred aspect of the present invention, the voltage-current conversion circuit to make up the voltage detection circuit outputs electric current corresponding to the detected charging voltage of the capacitor, and the variable current circuit outputs the base current corresponding to this electric current to the output transistor. Therefore, the base current corresponding to the charging voltage of the capacitor is supplied to the output transistor.
According to still another preferred aspect of the present invention, in the voltage-current conversion circuit to make up the voltage detection circuit, detection voltage given to the base terminal of the transistor for conversion is turned to an emitter voltage level-shifted by an amount equal to the base-emitter voltage VF. As a result, a collector current proportional to the emitter voltage is supplied to the transistor for conversion depending upon resistance value of the resistance for conversion. This collector current is turned to the base current of the output transistor via a current mirror circuit, which serves as the variable current circuit.
According to still another preferred aspect of the present invention, when it is designed using IC, the resistance for conversion is made of chromium silicon having low temperature coefficient. As a result, even when the IC is used under environmental condition with high temperature variation such as the condition in an automobile, it is possible to supply the base current adequately to the output transistor.
According to still another preferred aspect of the present invention, a starting circuit (such as resistance element) is connected between the input terminal of the voltage-current conversion circuit and the power line. As a result, even when the input terminal of the voltage-current conversion circuit has the same node as the output terminal of the current mirror circuit in the drive circuit, it is possible to start and drive the current mirror circuit.
According to still another preferred aspect of the present invention, an offset voltage generating circuit is connected between the input terminal of the voltage-current conversion circuit and the voltage detecting point. As a result, the voltage at the base terminal of the transistor for conversion is increased by an amount of the offset voltage compared with the voltage at the voltage detecting point. Then, the electric current corresponding to the offset voltage is continuously supplied to the transistor for conversion and to the current mirror circuit, and the current mirror circuit can be maintained at the condition of starting. Also, it is possible to reduce waveform distortion of the output voltage, which is likely to occur when the charging voltage of the capacitor is low (e.g. near 0 V).
According to still another preferred aspect of the present invention, the voltage-current conversion circuit of the voltage detection circuit switches over the current value to be outputted to a plurality of stages according to the result of comparison of the detection voltage with the reference voltage, and the variable current circuit outputs the base current corresponding to this electric current to the output transistor. Therefore, the base current corresponding to the charging voltage of the capacitor is supplied to the output transistor.
Further, according to still another preferred aspect of the present invention, a comparator circuit (e.g. comparator) outputs a voltage based on the result of comparison of the detection voltage with the reference voltage. This voltage is given to the base terminal of the transistor for conversion. It is level-shifted by an amount equal to the voltage VF and is turned to an emitter voltage. A collector current proportional to the emitter voltage is supplied to the transistor for conversion. This collector current is turned to the base current of the output transistor via the current mirror circuit, which serves as the variable current circuit.
Also, according to still another preferred aspect of the present invention, the charge-and-discharge circuit is provided with a first and a second constant current circuits for charging and discharging the capacitor. As a result, charging and discharging can be carried out at constant electric current, and it is possible to generate a voltage increased or decreased at a constant slew rate, e.g. a trapezoidal wave voltage.